1. Technical Field of the Invention
The present invention relates to the recovery of hydrocarbons from natural gas. More particularly, the invention relates to the recovery of ethane, propane, and heavier compounds from gas streams utilizing a lean reflux stream.
2. Description of the Related Art
Valuable hydrocarbon components, such as ethane, ethylene, propane, propylene and heavier hydrocarbon components, are present in a variety of gas streams. Some of the gas streams are natural gas streams, refinery off gas streams, coal seam gas streams, and the like. In addition these components may also be present in other sources of hydrocarbons such as coal, tar sands, and crude oil to name a few. The amount of valuable hydrocarbons varies with the feed source. The present invention is concerned with the recovery of valuable hydrocarbon from a gas stream containing more than 50% methane and lighter compounds [i.e., nitrogen, carbon monoxide (CO), hydrogen, etc.], ethane, and carbon dioxide (CO2). Propane, propylene and heavier hydrocarbon components generally make up a small amount of the overall feed. Due to the cyclical fluctuation of prices of natural gas and corresponding natural gas liquids (NGL), i.e., the hydrocarbon components recovered from natural gas in liquid form, processes need to be developed that can run under both ethane recovery and propane recovery modes of operation. In addition, these processes need to be easy to operate and be efficient in order to maximize the revenue generated from the sale of NGL.
Several processes are available to recover hydrocarbon components from natural gas. These include refrigeration processes, lean oil processes, refrigerated lean oil processes and cryogenic processes. Of late, cryogenic processes have largely been preferred over other processes due to better reliability, efficiency, and ease of operation. Depending on the desired hydrocarbon components to be recovered, i.e. ethane and heavier components or propane and heavier components, the cryogenic processes are different. Typically, ethane recovery processes employ a single tower with reflux to increase recovery and make the process efficient, such as illustrated in U.S. Pat. No. 4,519,824 issued to Huebel; U.S. Pat. No. 4,278,457 issued to Campbell et al.; and U.S. Pat. No. 4,157,904 issued to Campbell et al.
However, when ethane prices are not favorable to justify ethane recovery in NGL, the ethane component needs to be rejected while still maintaining high propane and heavier component recovery. The single tower ethane recovery schemes mentioned above can reject ethane, but these schemes start to loose propane and heavier components as ethane rejection increases. In order for a process to easily reject ethane while recovering most of the propane and heavier components, a propane recovery process needs to be considered. Most high propane recovery processes consist of two towers, typically with one tower being an absorber column and the other being a deethanizer column. This two tower scheme rejects essentially all the ethane, while recovering a large portion of propane and heavier components. In order for the process to recover 99% of the propane and heavier components, the absorber column is provided with an additional lean reflux stream. This lean reflux stream is responsible for essentially complete recovery of propane. An example two-tower scheme for propane recovery can be seen in U.S. Pat. No. 5,771,712 issued to Campbell et al (hereinafter referred to as “the '712 patent”).
The '712 patent describes processes that use a two tower scheme for propane recovery with FIGS. 4 and 5 of the '712 patent being the preferred embodiments. The propane recovery schemes described in the '712 patent can be more economical to build. For example, FIG. 6 of the '712 patent is an embodiment that eliminates the reflux system from the preferred embodiments to reduce capital cost. However, significant efficiency is lost in the process and this scheme has a limitation in that the propane recovery is less than 99%.
A need exists for a propane recover process that is capable of recovering greater than 99% propane without an increase in compression power, when compared with comparable processes. It would be desirable if the process could be constructed so that there is a reduction in capital costs, such as by eliminating a reflux system.